1. Field of Invention
This invention relates to novel organic phosphorus compounds. The compounds have general utility as surfactants and particular utility as an additive to perfluoropolyether compositions, including oil and grease lubricants, to inhibit corrosion and rust of metals in contact with the compositions.
2. Description of Related Art
Perfluoropolyethers have excellent thermal and oxidative stability and are used as greases, hydraulic fluids, and oils for service under extreme or demanding conditions wherein more conventional oils and greases are not adequate, for instance at temperatures above 260.degree. C. and up to 370.degree. C. to 425.degree. C., depending on the particular oil or grease. These oils and greases are available from a number of commercial sources, including E. I. du Pont de Nemours that markets them under the Krytox.RTM. trademark.
While the perfluoropolyethers per se are highly stable, they are highly permeable to oxygen and moisture. Thus, the oil and grease do not form an effective barrier against rust and corrosion of metallic parts they contact. As used herein, the term "corrosion" refers to the oxidation of metal in contact with oxygen, and the term "rust" refers to the oxidation of metal in contact with water.
Sodium nitrite conventionally has been added as a corrosion inhibitor to some perfluoropolyether greases. Sodium nitrite is not soluble in perfluoropolyether oils. In the greases, it is present as a particulate dispersion. Ideally, a rust and corrosion inhibitor would be present in solution to provide better protection and to eliminate the potential problem of separation from the oil or grease.
Many organic phosphorus compounds have been suggested in the patent literature as rust and/or corrosion inhibitors for perfluoropolyether oils and greases. For example, U.S. Pat. No. 3,306,855 to Borecki proposes a perfluoroalkyl ether phosphate containing at least one acidic hydrogen, and U.S. Pat. No. 5,550,277 to Paciorek et al., proposes a variety of fully and partially esterified phosphates and phosphates, some of which are acidic.
Other candidate rust and corrosion inhibitors proposed in the art are very difficult or expensive to synthesize. For example, some of the proposed additives contain large quantities of costly perfluoroalkyl substituents. Others, such as some of the partially esterified phosphates proposed by U.S. Pat. No. 5,550,277, require formation of an intermediate fluoroether-substituted phenol that is very difficult to synthesize in a commercial manufacturing process.
The use of acidic rust and corrosion inhibitors is contraindicated in admixture with perfluoropolyether greases containing the common sodium nitrite inhibitor, since the acidic group may react with the sodium nitrite to generate noxious nitrogen oxides. Since the presence of greases containing sodium nitrite inhibitors has been pervasive, and they are expensive to replace, there is a high probability that new greases and oils will be placed in existing equipment lubrication reservoirs containing sodium nitrite.
Thus, there is an ongoing need for new corrosion and rust inhibitor additives for perfluoropolyether compositions, including oils and greases.